On the ground: Virtual wallet
The idea of a virtual wallet isn't new. From online payment systems like PayPal, to credit and debit cards with a chip that will store cash amounts have been a reality for a while. But in Japan, where most new technologies seem to appear these days, they have integrated a virtual wallet to cell phones.
The system is called Edy, and people can load up money to their virtual wallet by going to a recharge unit or by using their phone to do the transfer online. Then when going to a store they simply put their phone near the store machine and the transfer is done. It's one more item added to the convergence wagon, and it has some good advantages like convenience. The biggest obstacle to this, like any of the previous virtual wallet solutions, will be consumer acceptance.